Field of the Invention
The invention relates to the production of a fibre composite component part based on steel and polyurethane.
Discussion of the Background
Fibre composite component part refers to a component part which is a component part of a machine, of a land-, air-, space- or water-craft, of an apparatus, of an installation or of an appliance and which is constructed of different, indissolubly interconnected materials subject to the proviso that at least one material takes the form of fibres and at least one material takes the form of a matrix surrounding said fibres. The shape of this fibre composite component part is substantially the shape that is determined by its intended use. As a result, the fibre composite component part is virtually ready to install/use and, apart from minor secondary finishing, requires no further significant changes in shape before installation/use.
The present fibre composite component part comprises the materials steel, polyurethane as well as the fibre material.
Steel is to be understood as referring to a group of iron materials whose mass content of iron is greater than that of every other element present. The carbon content is below 2% by weight.
Polyurethane is to be understood as referring to a group of polymers that are obtained by reaction of diisocyanates with polyols. Polyurethane is frequently abbreviated to PU or PUR.
Compared with component parts produced from homogeneous materials, the production of fibre composite component parts has one special feature: this is that, while a homogeneous material is generally already in existence before the component part is produced, a fibre composite material is only formed in the course of the actual fibre composite component part being produced. Therefore, the production of a fibre composite component part is always directly tied to the production of the related fibre composite material.
Similarly, the design and construction of the fibre composite material must always take account of the later loading on the fibre composite component part consisting of said fibre composite material in order that the loads are as intended directed via the matrix into the fibres and transmitted by them.
The materials science of manufacturing fibre composite component parts is accordingly much more tricky than the materials science of producing component parts from homogeneous materials.
This is particularly true when a fibre composite component part is to be constructed from rather dissimilar materials, for example when carbon fibre, which is inorganic, is to be embedded in an organic-type polymer matrix and the latter is to be additionally surrounded with metallic outer layers.
A fibre composite component part of this type promises to combine the advantages of the particular individual materials to obtain overall a component part of very high mechanical strength, good thermal insulation, vibration damping and corrosion resistance coupled with very low weight.
Producing such a component part is a supreme technical challenge, since the specific expertise of organic and inorganic chemistry, of metal processing and regarding the in-service conditions expected for the component part must be combined.
It is particularly noteworthy that the industrial manufacture of polymers differs greatly from metal production and processing, especially with regard to workflow organization. The industrial practice of the method developed for producing a fibre composite component part based on steel and polyurethane can accordingly only be successful if said method can be integrated in both the workflow of a chemical manufacturing operation and the workflow of a metal-processing operation.
It will be appreciated that numerous examples of organometallic fibre composite component parts are already found in the related art:
DE10158491A1 for instance discloses a metal-polyurethane laminate constructed of two layers of electrogalvanized sheets of steel which are arranged either side of a polyurethane layer which may incorporate filler and reinforcing agents. The polyurethane is cured between the steel sheets to form a plate-shaped composite component part possessing high mechanical strength. This method of production is disadvantageous in that it always leads to a planar laminate, the utility of which is limited by its flatness. True, a peel test is described in the form of a bending test wherein the laminate is bent by 90° and back again. However, an elevated application of force is needed for this forming operation, since at this stage the polyurethane is already thermoset and so the composite component part has already attained its full strength. As to whether the formed laminate is shape-stable and does not strive to return to its original flat shape, DE10158491A1 is silent. This method is incidentally also disadvantageous because of the rapid reaction of the isocyanate/polyol components to form the polyurethane, making it necessary to produce this composite component part under time pressure. Nor is a fibre composite component part concerned here, but a sandwich of polymer and sheet metal.
A similarly constructed steel-PU composite component part is known from DE19914420A1. However, the thickness of the metal is an immense 2 to 20 mm, so the steel here is in the form of heavy plate which, unlike fine sheet (thickness <3 mm), is not coilable. A laminate of this type is therefore impossible to process with a conventional panel press of the type used in automotive construction for example. This is why ship or bridge building is instead the intended destination for the laminate.
The composite structure known from WO98/21029A1 is similarly intended for ship building. It comprises two outer, mutually spaced-apart plates of steel either side of a void space packed with an elastomeric polyurethane. The elastomer dampens vibrations in the metal structure. Again, this is not a fibre composite component part. Moreover, the elastomer is not introduced into the void space until after finalizing the steel structure. WO2009/009207A2 discloses a method of producing a fibre composite component part based on a metal and a thermoplastic polyurethane. To this end, an already fibre-containing thermoplastic polyurethane raw material is melted and press formed between metal sheets under heat. This method is advantageous in that the fibre composite component part may also assume a non-flat shape and requires few processing steps. This method is disadvantageous in that the fibres, in the form of short fibre, are already present in the polyurethane raw material, thus preventing intentional alignment of the fibres in the direction of the later transmission/distribution of forces/stresses. What is made, on the contrary, is a fibre composite component part comprising randomly oriented fibre.
The sandwich panel known from DE102012106206A1 covers a distinctly wider spectrum of possible uses. It comprises two metallic outer sheets either side of a core layer of fibre-containing polyurethane. This sandwich panel has good plastic formability; even deep-drawability into three-dimensional carbody parts. One disadvantage here is the aqueous formulation of the reactive polyurethane mixture, so water vapour has to escape from the laminate during the processing operation. Unless all the water is successfully removed, internal corrosion of the steel sheets is likely. In addition, the reported polyurethane composition is highly reactive, so shaping has to take place within a tight processing window. Overall it appears to be questionable whether the method described in DE102012106206A1 is capable of producing a steel-PU composite component part on an industrial scale and in a commercially viable manner.
The problem of the tight processing window for high-reactive polyurethane mixtures was solved in DE102009001806A1 by incipiently crosslinked yet storage-stable prepregs, which are thermoplastically formable and are fully crosslinked thermoset thereafter. The particular advantage of these prepregs—that is, woven or non-crimp fabric coated with matrix material—resides in their stability in storage. As a result, they are sufficiently long storable and transportable in the incipiently crosslinked, thermoplastic state. So, while the prepregs are manufactured in a first facility, their forming into a shaped article and the full crosslinking thereof into the fibre composite component part may be carried out elsewhere, at a remote site. This allows new degrees of freedom in the organization of fibre composite component part production and particularly also competency-based division of labour across two or more production facilities. However, these storage-stable prepregs are disadvantageous in that they are not unreservedly useful for lamination with steel. More particularly, they fail to achieve a sufficient level of steel-polymer bonding for safely forming a laminate obtained therefrom, as Example 0 will demonstrate herein below.
Consequently, no method has been described to date for obtaining fibre composite component parts from steel and a polyurethane on an industrial scale and in a commercially viable manner.
It is an object of the present invention to devise such a method.